Flicker noise occurs in transistors that form a current source. The flicker noise is caused by defects in the transistors and the defects affect movement of electrons and holes so that the output of the current source becomes abnormal in a random manner. It is difficult to predict the fluctuations in the current source output caused by the flicker noise. In addition, the time necessary for a terminal voltage of a capacitor to reach a reference voltage fluctuates due to the flicker noise, and as a result, the oscillation frequency may fluctuate.